Help ;LL i think my brother smokes weed <3?

i love my brother to pices the problem is that he has just turned 18 and i figured out that he smokes weed im only 13 and all my life i used to look up to him but now i dont. it really upsets me to think he is doing this i dont understand if he is harming himself or any thing?? i found out he smokes weed as i was stood behind he and he was talking and he said "yeh i had to go in the shed t get the weed " i dont think he new i was there but i was! it wasent our shed . i really want him to stop you see i think he is ruining his life he didint get good gcse and he has no job he is pon the dole. he has had a job but then he got fired because he diddint smile. i am really worried about him and it makes me cry i think my parents have an idea but i want to mkae him stop alltogther some one help me please !!! ;'(( x